The present invention relates to devices for downhole drilling and, more particularly, to steerable motor drives with formation evaluation capability.
Downhole drilling devices of the positive displacement type are well known. For example, U.S. Pat. No. 5,135,059, which is assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, discloses a downhole drill which includes a housing, a stator having a helically contoured inner surface secured within the housing and a rotor having a helically contoured exterior surface disposed within the stator. Drilling fluid (e.g., drilling mud) is pumped through the stator which causes the rotor to move in a planetary type motion about the inside surface of the stator. A drive shaft is connected to the rotor via a flexible coupling to compensate for the eccentric movement of the rotor. Other examples of downhole drilling devices are disclosed in U.S. Pat. Nos. 4,729,675, 4,982,801 and 5,074,681 the disclosure of each of which are incorporated herein by reference.
Formation evaluation tools assist operators in identifying the particular geological material through which a drill is passing. This feedback of information is used by operators to direct the drilling of a well, through, in the case of a horizontal well, a desired layer or stratum without deviating therefrom. These tools have employed several techniques in the past which have been used independently and/or in some combination thereof. Formation resistivity, density and porosity logging are three well known techniques. One resistivity measuring device is described in U.S. Pat. No. 5,001,675 which is assigned to the assignee hereof and is incorporated herein by reference. This patent describes a dual propagation resistivity (DPR) device having one or more pairs of transmitting antennas spaced from one or more pairs of receiving antennas. Magnetic dipoles are employed which operate in the mf and lower hf spectrum. In operation, an electromagnetic wave is propagated from the transmitting antenna into the formation surrounding the borehole and is detected as it passes by the two receiving antennas. The phase and the amplitude are measured in a first or far receiving antenna which is compared to the phase and amplitude received in a second or near receiving antenna. Resistivities are derived from the phase differences and the amplitude ratio of the received signals. The formation evaluation of DPR tool communicates the resistivity data and then transmits this information to the drilling operator using mud pulse telemetry. Other examples of DPR units are disclosed in U.S. Pat. Nos. 4,786,874, 4,575,681 and 4,570,123.
Formation density logging devices, such as that described in U.S. Pat. No. 5,134,285 which is assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, typically employ a gamma ray source and a detector. In use, gamma rays are emitted from the source, enter the formation to be studied, and interact with the atomic electrons of the material of the formation and the attenuation thereof is measured by the detector and from this the density of the formation is determined.
A formation porosity measurement device, such as that described in U.S. Pat. No. 5,144,126 which is assigned to the assignee hereof and fully incorporated herein by reference, include a neutron emission source and a detector. In use, high energy neutrons are emitted into the surrounding formation and the detectors measure neutron energy depletion due to the presence of hydrogen in the formation. Other examples of nuclear logging devices are disclosed in U.S. Pat. Nos. 5,126,564 and 5,083,124.
In directional drilling (e.g., a horizontal well), it is desired to maintain the wellbore within the pay zone (i.e., a selected bed or stratum) for as long as possible since the desired raw material may be laterally displaced throughout the strata. Therefore, a higher recovery of that material occurs when drilling laterally through the stratum. The drill bit is typically steered through the pay zone by rotating the drill collar which, because of a small bend in the lower portion of the drill collar, will turn the drill bit into a different direction. However, the distance between the DPR sensor and the bit (e.g., generally in excess of four feet) requires the wellbore to be drilled at a minimal angle with respect to the longitudinal direction of the pay-zone, otherwise the drill bit may enter a different zone long before the DPR sensor would recognize that fact. In the situation where the adjacent zone includes water, a potential problem becomes more readily apparent.
In drilling apparatus all three of these tools for evaluating a formation may be employed downhole in a drill housing or segment. The most effective at determining whether there is a change in strata ahead of the drill bit, e.g., oil water contact, is the resistivity logging device. Oil, water contact for example has a resistivity change of 100 ohms per meter away from the low resistance side of the contact point. However, in the past, excessive spacing between the resistivity measuring (or logging) device and the bit prevented accurate readings as previously discussed. Unfortunately, the resistivity measuring device could not be located close to the bit because of the use of conventional mud motors and stabilization displacing the resistivity sensor 25' from the bit at minimum.